1. Field of the Invention
The present invention relates to a circuit technology for semiconductor devices. Particularly, it relates to a technology that can be effectively applied to a semiconductor device with components that can be fabricated on a semiconductor substrate chip, including an integration circuit with a time constant on the millisecond order, a decrement circuit, and a semiconductor device with a soft shutting-down function. The invention also relates to an ignition device (to be hereafter referred to as “an ignitor”) for automotive engines to which the semiconductor device is applied.
2. Background Art
The following is an analysis of ignitors for automotive engines conducted by the inventors.
As ignitors for automotive engines, distributorless ignition systems are increasingly replacing the conventional distributor-employing systems for the purpose of saving energy. In the distributorless ignition systems, each cylinder of the internal combustion engine is provided with an ignitor and an ignition coil. The ignitor conducts switching control of the current on the primary side of the ignition coil such that a high voltage of several tens of thousands of volts is generated on the secondary side of the ignition coil. The high voltage causes an ignition plug to produce discharge while combustion is controlled within the cylinder. The switching device for the ignitor is increasingly comprised of an insulated gate semiconductor device, instead of the conventional bipolar transistors.
Such an ignitor is a power device for switching the current through the ignition coil in response to an ignition-control signal from an engine control unit (to be hereafter referred to as “an ECU”). In a normal operation, the control signal from the ECU is a pulse signal on the order of several milliseconds. However, the pulse width of the control signal from the ECU might be rendered into a continuously applied signal due to one cause or another. The current on the primary side of the ignition coil increases in accordance with a slope that is determined by the inductance L and the applied voltage V (di/dt=V/L). Therefore, the continuously applied signal would cause a current to flow continuously on the primary side of the ignition coil that would exceed an acceptable value, thereby damaging or burning out the ignition coil and the switching device. Ignitor devices have been developed that are fitted with a current-limiting function for preventing excess over a predetermined current limitation value. However, even if the value of the continuous current is controlled by the current-limiting function, the current would produce an amount of heat corresponding to the power as the product of voltage and current. The heat could lead to a thermal runaway or burning out of the ignitor. To prevent this problem, devices have also been developed that have a function for automatically shutting themselves down if their temperature exceeds a predetermined set value.
For example, Patent Document 1 discloses that current is forcibly terminated upon the detection of abnormal heating. The disclosed device comprises a pulse generating circuit, a counter circuit, and a step-waveform generating circuit, which are controlled in the following manner for preventing an erroneous ignition of the ignition plug upon forcible shutdown. The pulse generating circuit acts as an oscillator, while the counter circuit acts as a timer for causing a signal to be produced at desired periods based on a signal fed from the counter circuit in the previous stage. When overheating is detected, the output voltage of the step waveform generating circuit is controlled to produce a step voltage at the aforementioned periods. The step waveform is used to control a compulsory shutdown circuit such that the main current of the switching device is shut down in a stepwise manner. The step periods and the amount of current by which the main current is reduced per period are determined such that the voltage generated on the secondary side of the ignition coil does not exceed the value at which the ignition plug starts to produce spark discharge. In this publication, the periods are set to be on the order of 2 milliseconds so as to prevent erroneous ignition.
Patent Document 1: JP Patent Publication (Kokai) No. 2001-248529 A